The present invention relates generally to automatic gain control (AGC) for use in radio-frequency (RF) communications systems. More specifically, but without limitation thereto, the present invention relates to an automatic gain control with digital filtering for mitigating dynamic range reduction due to out-of-band interference.
Typical radio frequency communications signals are divided into a number of channels within a signal band centered around a carrier frequency. For example, 10 channels each 5 megahertz (MHz) wide can fit into a 50 MHz wide frequency band centered at a carrier frequency of, for example, 2 gigahertz (GHz). About 99% of the signal in each channel is contained in a band of frequencies about 4.4 MHz wide, ordinarily leaving a gap between adjacent channels to avoid mutual interference. During transmission of the signal, however, signal from one channel may cross over into other channels (or bands) causing out-of-band interference. Much time, effort, and ingenuity has been devoted to the problem of filtering out this out-of-band interference.
One of the difficulties encountered in signal filtering is that radio frequency signals in the gigahertz range are much more difficult to filter than signals at lower frequencies. One approach to solving this filtering problem is to translate the radio frequency signal to an intermediate frequency (IF) signal. For example, if a 5 MHz channel is transmitted on a carrier in the gigahertz range from 1.995 GHz to 2.000 GHz, the carrier may be translated to an intermediate frequency at some lower frequency, for example, 700 MHz. The 5 MHZ channel would then occupy from 695 MHz to 700 MHz. At this lower frequency, filtering is relatively straightforward.
Analog filters in the radio frequency, intermediate frequency, and baseband ranges are typically used to mitigate out-of-band interference. Disadvantageously, analog filters can be expensive, especially for the higher frequencies.
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